Why am I getting a "void could not be converted to int" error with JOptionPane.showInputDialog?

Question

I have this assignment for school where I have to use JOptionPane to calculate the area of different shapes, but when I use JOptionPane.showMessageDialog, it turns red and the error code says that void cannot be converted to int. Can someone help me figure out how to use JOptionPane.showMessageDialog?

public static void areaTriangle(int base, int height)
{
    System.out.println(0.5 * base * height);
}

public static void areaCircle(int radius, double area)
{
    area = Math.PI * (radius * radius);
    System.out.println(area);
}

public static void areaRectangle(int length, int width)
{
    System.out.println(length * width);
}

public static void calcArea()
{
    System.out.println("What type of area do you want to calculate?");
    inputStr = JOptionPane.showInputDialog("Type 1 for triangle, 2 for circle, 3 for rectangle, and 0 for none, then press ENTER.");
    int type = Integer.parseInt(inputStr);

    if (type == 0)
    {
        return;

    }
    else if (type == 1)
    {
        inputStr = JOptionPane.showInputDialog("Enter the measurement of the base, then press ENTER.");
        int base = Integer.parseInt(inputStr);
        inputStr = JOptionPane.showInputDialog("Enter the measurement of the height, then press ENTER.");
        int height = Integer.parseInt(inputStr);
        int areaT = areaTriangle (base, height);
        JOptionPane.showMessageDialog(null, "The area of the triangle is: " + areaT);

    }
    else if (type == 2)
    {
        inputStr = JOptionPane.showInputDialog("Enter the measurement of the radius, then press ENTER.");
        int radius = Integer.parseInt(inputStr);
        double area = Math.PI * (radius * radius);
        int areaC = areaCircle (radius, area);
        JOptionPane.showMessageDialog(null, "The area of the cicle is: " + areaC);

    }
    else if (type == 3)
    {
        inputStr = JOptionPane.showInputDialog("Enter the measurement of the length, then press ENTER.");
        int length = Integer.parseInt(inputStr);
        inputStr = JOptionPane.showInputDialog("Enter the measurement of the width, then press ENTER.");
        int width = Integer.parseInt(inputStr);
        int areaR = areaRectangle(length, width);
        JOptionPane.showMessageDialog(null, "The area of the rectangle is: " + areaR);
    }

}

public static void main(String[] args)
{
    calcArea();
}

Answer 1

Your problem has nothing to do with the JOptionPane and everything to do with your area* methods

First you declare all your methods as void...

public static void areaTriangle(int base, int height) {
    System.out.println(0.5 * base * height);
}

public static void areaCircle(int radius, double area) {
    area = Math.PI * (radius * radius);
    System.out.println(area);
}

public static void areaRectangle(int length, int width) {
    System.out.println(length * width);
}

But then you try and assign a return result to a variable when you call them...

int areaT = areaTriangle(base, height);
//...
int areaC = areaCircle(radius, area);
//...
int areaR = areaRectangle(length, width);

Which simply makes no sense.

Based on what you seem to be trying to do, you need to change your area* methods to return a value of some kind...

public static double areaTriangle(int base, int height) {
    return 0.5 * base * height;
}

public static double areaCircle(int radius, double area) {
    return Math.PI * (radius * radius);
}

public static int areaRectangle(int length, int width) {
    return length * width;
}

Then you can assign the return values from the methods to some variables...

double areaT = areaTriangle(base, height);
//...
double areaC = areaCircle(radius, area);
//...
int areaR = areaRectangle(length, width);